drm/radeon/dp: Fix long-standing bug in Display Port handling.

drm/radeon/dp: Fix long-standing bug in Display Port handling.

This fixes EDID information fetching from monitor through aux/iic
when using Display Port and no longer requires providing binary
EDID block with drm.edid_firmware="DP-1:edidfw_MONITOR" tunable.

From testing/disecting looks like bug was from initial radeonkms port.

While there, add hpd (hot plug detection) pin assignment for future
reference when common drm i2c/iic over aux will be implemented.

Changes intended only for Display-Port.
Note that currently on DP cable remove or monitor power off when monitor
is plugged-in DP connector stays in disconnected state (DP-only quirk).

Tested-on: R7 360 BONAIRE w/ HP LA DP-1(1920x1200), HP LP DVI(1600x1200)
           HD5770 JUNIPER w/ DELL P2416D DP-1(2560x1440) and
                                         HDMI(1920x1200)[card limit]